Date:   Wed, 22 Mar 2023 09:26:51 +0000
From:   James Clark <james.clark@....com>
To:     Mike Leach <mike.leach@...aro.org>
Cc:     coresight@...ts.linaro.org,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Leo Yan <leo.yan@...aro.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/9] coresight: Fix CTI module refcount leak by making
 it a helper device



On 21/03/2023 15:12, Mike Leach wrote:
> Hi James
> 
> On Fri, 10 Mar 2023 at 16:06, James Clark <james.clark@....com> wrote:
>>
>> Changes since v1:
>>
>>  * Don't dereference handle in tmc_etr_get_buffer() when not in perf mode.
>>  * Fix some W=1 warnings
>>  * Add a commit to rename child/output in terms of local/remote
>>
>> -------------------
>>
>> Currently there is a refcount leak in CTI when using system wide mode
>> or tracing multithreaded applications. See the last commit for a
>> reproducer. This prevents the module from being unloaded.
>>
>> Historically there have been a few issues and fixes attempted around
>> here which have resulted in some extra logic and a member to keep
>> track of CTI being enabled 'struct coresight_device->ect_enabled'.
>> The fix in commit 665c157e0204 ("coresight: cti: Fix hang in
>> cti_disable_hw()") was also related to CTI having its own
>> enable/disable path which came later than other devices.
>>
>> If we make CTI a helper device and enable helper devices adjacent to
>> the path we get very similar enable/disable behavior to now, but with
>> more reuse of the existing reference counting logic in the coresight
>> core code. This also affects CATU which can have a little bit of
>> its hard coded enable/disable code removed.
>>
>> Enabling CATU on the generic path does require that input connections
>> are tracked so that it can get its associated ETR buffer.
>>
>> Applies to coresight/next (669c4614236a7) but also requires the
>> realloc_array patch here [1].
>>
>> Also available in full here [2].
>>
>> [1]: https://lore.kernel.org/linux-arm-kernel/20230306152723.3090195-1-james.clark@arm.com/
>> [2]: https://gitlab.arm.com/linux-arm/linux-jc/-/tree/james-cs-cti-module-refcount-fix-v2
>>
>> James Clark (9):
>>   coresight: Use enum type for cs_mode wherever possible
>>   coresight: Change name of pdata->conns
>>   coresight: Rename nr_outports to nr_outconns
>>   coresight: Rename connection members to allow for input connections
>>   coresight: Dynamically add connections
>>   coresight: Store in-connections as well as out-connections
>>   coresight: Refactor out buffer allocation function for ETR
>>   coresight: Enable and disable helper devices adjacent to the path
>>   coresight: Fix CTI module refcount leak by making it a helper device
>>
>>  drivers/hwtracing/coresight/coresight-catu.c  |  34 +-
>>  drivers/hwtracing/coresight/coresight-core.c  | 312 +++++++++++-------
>>  .../hwtracing/coresight/coresight-cti-core.c  |  56 ++--
>>  .../hwtracing/coresight/coresight-cti-sysfs.c |   4 +-
>>  drivers/hwtracing/coresight/coresight-cti.h   |   4 +-
>>  drivers/hwtracing/coresight/coresight-etb10.c |   3 +-
>>  .../coresight/coresight-etm3x-core.c          |   6 +-
>>  .../coresight/coresight-etm4x-core.c          |   6 +-
>>  .../hwtracing/coresight/coresight-platform.c  | 178 +++++++---
>>  drivers/hwtracing/coresight/coresight-priv.h  |   9 +-
>>  drivers/hwtracing/coresight/coresight-stm.c   |   6 +-
>>  drivers/hwtracing/coresight/coresight-sysfs.c |   9 +-
>>  .../hwtracing/coresight/coresight-tmc-etf.c   |   2 +-
>>  .../hwtracing/coresight/coresight-tmc-etr.c   |  89 ++---
>>  drivers/hwtracing/coresight/coresight-tmc.h   |   2 +
>>  drivers/hwtracing/coresight/coresight-tpdm.c  |   4 +-
>>  drivers/hwtracing/coresight/coresight-tpiu.c  |   3 +-
>>  drivers/hwtracing/coresight/coresight-trbe.c  |   3 +-
>>  drivers/hwtracing/coresight/ultrasoc-smb.c    |   3 +-
>>  drivers/hwtracing/coresight/ultrasoc-smb.h    |   2 +-
>>  include/linux/coresight.h                     | 109 +++---
>>  21 files changed, 530 insertions(+), 314 deletions(-)
>>
>> --
>> 2.34.1
>>
> 
> Looking at this overall - given that the only use of the in_conn is to
> reference the connecting device from the helper,
> 
> i.e. coresight-catu.c:405:           tmp = csdev->pdata->in_conns[i].remote_dev;
> 
> would it not be simpler to :
> 
> a)
> in coresight_connection add a field:
> 
>   struct coresight_device *origin_dev;
> 
> which mimics the origin / target model we already have in coresight_sysfs_link
> then
> 
> b) the in_conns could simply be references to out_conn object from
> origin_dev, rather than a complete coresight_connection  with reversed
> values, thus simplifying the in_conns handling code, and removing the
> unused reversed feilds in the current in_conn object.
> 
> e.g. tmp = csdev->pdata->in_conns[i]->origin_dev
> 
> The remainder of the code would remain much the same, just adjusted
> for in_conns as refs rather than independent conn objects

This was actually my first implementation because I also thought it
would be simpler. Unfortunately it didn't work because the realloc means
that storing references to the connections is impossible as they are
moved if the array needs to grow.

Based on Suzuki's and Jinlongs comments about missing some of the input
connections some of the time, now I've made a single place for fixing up
output connections (currently there are two) and put the new input code
there. So V3 will be a bit simpler in that any time a connection is made
the input is made at the same time so it's not conceptually any more
complicated than the current code.

> 
> Regards
> 
> 
> Mike
> 
> --
> Mike Leach
> Principal Engineer, ARM Ltd.
> Manchester Design Centre. UK

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